The present application relates generally to the monitoring of output power from a semiconductor laser diode, and more particularly to the monitoring of output power from a vertical cavity surfacing emitting laser (VCSEL) as used in a raster output scanning (ROS) system.
It is well known in the scanning art to use laser diodes to generate a coherent laser beam which is then optically shaped for use as a scanning beam in a ROS system. It is also known to use multiple laser diodes to create multiple beams with each individual beam being independently modulated by video signals and scanned onto the recording or image surface as modulated beams. For these multiple beam applications, it has been found advantageous to use the rays with closely spaced laser diodes. Typically, the laser diodes in a multiple beam system are individually addressable. Individually addressable diodes require that each diode have a separate current source that drives or modulates the diodes. In operation, each driver sends a current through the diodes sufficient to induce emission of laser light, and the amount of current the driver produces is determined, in part, by the physical input data driving the particular lasing element. Because different laser diodes have different types of power characteristics in response to a given driving current, it is also desirable to monitor the amount of output power from each laser diode individually.
In systems utilizing diodes having a front (main) facet and a back facet, it may be possible to monitor light escaping from the back facet of the diode. This approach makes it possible to monitor the light output of the diode without interfering or disturbing the useful light being emitted out of the front facet of the diode. An example of a method and apparatus for controlling the power output of multiple laser diodes utilizing radiation from the back facet is disclosed in U.S. Pat. No. 5,600,126, the disclosure of which is incorporated herein by reference in its entirety. Typically, however, back facet monitoring is accomplished using an internal photodiode rather than an external photodiode as disclosed in this patent.
However, many systems now use a VCSEL diode or an array of VCSEL diodes. These diodes do not emit light from a back facet, but rather only emit light from one surface or facet of the diode. Therefore, it is not possible to take advantage of light being emitted from a back facet. Typically, the VCSEL diode laser output is diverging and, therefore, requires the use of a collimating lens. The beam is then sent through an aperture for shaping, and is imaged by scanner optics. One option to monitor and control the output from a VCSEL diode is to place a beam splitter directly in the beam path and reflect a portion of the beam back onto a photodiode used for measuring and monitoring the VCSEL diode output. An example of a ROS system which reflects a portion of the collimated light outputs to impinge on one or two photodiodes configured to monitor and control the output of the VCSEL diodes is disclosed in U.S. Pat. No. 5,659,414, the disclosure of which is incorporated herein by reference in its entirety.
FIG. 5 shows one embodiment of a prior art ROS system which reflects a portion of a collimated beam for purposes of controlling VCSEL diodes. In this embodiment, an array 10 is comprised of laser diodes 12, 14. Two photodiodes 16, 18 are mounted on a front face of a heat sink 20 for sensing light impinging on the photodiodes. Output beams 22, 24 are collimated by a collimating lens 26. Positioned between the collimating lens 26 and a ROS optical system (not shown) is a partially reflecting beam splitter mirror 28. The mirror 28 is tilted at an appropriate angle to reflect part of the collimated beam back through the collimating lens 26 so as to reimage a portion of the light from the laser diodes 12, 14 onto the photodiodes 16, 18. Signals from the photodiodes 16, 18, which vary according to the intensity of the reflected light, are coupled to a feedback circuit 30, used to control the output of laser diodes 12, 14, thereby maintaining the output intensity at a predetermined level.
An aspect of the prior art embodiment is that the arrangement collects an appreciable fraction of the emitted light from laser diodes 12, 14 which could otherwise be beneficially used by the ROS scanning optic system. Also a large two-dimensional diode laser array and its associated integrated circuit chip restrict or prohibit the placement of photodiodes in close proximity in order to be able to collect sufficient back-reflected light as depicted in FIG. 5.